The present invention relates generally to a method and apparatus for orienting a plastic. More specifically, the present invention relates to a method for the preferably biaxial molecular orientation of plastic material and also to an arrangement for the realization of the method. The present invention also relates to a plastic material manufactured in accordance with the method.
Within the technique of packaging, polyethylene terephthalate (PET), or polyester as it is called more popularly, is used for the manufacture of packages for pressurized contents. The reason is that the polyester material has good mechanical strength characteristics which can be improved further if the material is molecular-oriented. A molecular orientation is achieved by stretching the material within a certain temperature range, which for polyester is 70.degree.-100.degree. C. If the temperature is too low it will be impossible on the whole to stretch the material without tearing the same, and if the temperature is too high only a thinning of the material will take place, but not the intended molecular orientation. If a stretching of the material is carried out within the specified temperature range, a molecular orientation is obtained, the extent of which depends partly on the temperature, and partly on the degree of stretching. As mentioned earlier, the mechanical characteristics, e.g. the ultimate tensile strength, are substantially improved by molecular orientation. A stretching of the material in one direction produces a so-called monoaxial orientation, that is to say the material will be molecular-orientated only in one direction, which means that the improved strength characteristics of the plastic material are obtained only in the direction the material was pulled. For many purposes a monoaxial stretching is sufficient, but e.g. for containers with pressurized contents, e.g. bottles or other packages for beer or refreshing beverages, the stresses in the material will be effective not only in one direction. This means that the improved strength characteristics in the direction the material was pulled cannot be utilized, because the material does not possess correspondingly good strength characteristics in the direction which is perpendicular to the direction of orientation. This difficulty can be overcome, however, if the material is oriented biaxially, that is to say in two directions right-angled to one another, to give a plastic material which in principle is of uniform strength.
On blowing polyester bottles, an expansion occurs not only in one direction, but the material is oriented biaxially, if the blowing is carried out within the temperature range specified earlier. Such blown polyester bottles have found much use for the packaging of e.g. refreshing beverages, but it is a disadvantage of these bottles, that an individual blowing of bottles brings about a low production capacity and hence an expensive product. Furthermore the orientation obtained in the blowing of these bottles is not uniform, but the orientation in the longitudinal direction is generally considerably less than the orientation in transverse direction of the bottle, and the degree of orientation within the different parts of the bottle, the bottom, the barrel part, the neck portion etc. show a wide variation of orientation.
Thus there is need for a relatively uniform biaxially oriented polyester material. Such a material can be produced, using the existing technique, in that a non-oriented material web is gripped with special grippers along its edge zones and is stretched in transverse direction of the web by the grippers being moved away from each other. At the same time the web is possibly stretched in its longitudinal direction. However such an orientation procedure produces a very high wastage (up to 40%) and is therefore expensive to use.
The present invention, however, suggests a method for the biaxial orientation with a relatively uniform degree of orientation of a plastic material, in particular polyester material. The invention is characterized in that a tube of non-oriented, or previously longitudinally oriented, plastic material is introduced into a container filled with heated liquid. Preferably a heated liquid under pressure is introduced into the interior of the tube which, owing to the internal pressure prevailing in the tube, is thereby made to expand, preferably in transverse direction of the tube, at the same time as the part of the tube situated in the container, which has been conditioned through heating by the heated liquid, is orientation-stretched in its longitudinal direction.